netdev - Re: [PATCH] virtio-net: disable delayed refill when pausing rx

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Message-ID: <4195db62-db43-4d61-88c3-7a7fbb164726@gmail.com>
Date: Wed, 9 Apr 2025 13:44:07 +0700
From: Bui Quang Minh <minhquangbui99@...il.com>
To: Jason Wang <jasowang@...hat.com>
Cc: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
 "Michael S . Tsirkin" <mst@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 Eugenio Pérez <eperezma@...hat.com>,
 "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
 virtualization@...ts.linux.dev
Subject: Re: [PATCH] virtio-net: disable delayed refill when pausing rx

On 4/8/25 14:34, Jason Wang wrote:
> On Mon, Apr 7, 2025 at 10:27 AM Bui Quang Minh <minhquangbui99@...il.com> wrote:
>> On 4/7/25 08:03, Xuan Zhuo wrote:
>>> On Fri,  4 Apr 2025 16:39:03 +0700, Bui Quang Minh <minhquangbui99@...il.com> wrote:
>>>> When pausing rx (e.g. set up xdp, xsk pool, rx resize), we call
>>>> napi_disable() on the receive queue's napi. In delayed refill_work, it
>>>> also calls napi_disable() on the receive queue's napi. This can leads to
>>>> deadlock when napi_disable() is called on an already disabled napi. This
>>>> scenario can be reproducible by binding a XDP socket to virtio-net
>>>> interface without setting up the fill ring. As a result, try_fill_recv
>>>> will fail until the fill ring is set up and refill_work is scheduled.
>>> So, what is the problem? The refill_work is waiting? As I know, that thread
>>> will sleep some time, so the cpu can do other work.
>> When napi_disable is called on an already disabled napi, it will sleep
>> in napi_disable_locked while still holding the netdev_lock. As a result,
>> later napi_enable gets stuck too as it cannot acquire the netdev_lock.
>> This leads to refill_work and the pause-then-resume tx are stuck altogether.
> This needs to be added to the chagelog. And it looks like this is a fix for
>
> commit 413f0271f3966e0c73d4937963f19335af19e628
> Author: Jakub Kicinski <kuba@...nel.org>
> Date:   Tue Jan 14 19:53:14 2025 -0800
>
>      net: protect NAPI enablement with netdev_lock()
>
> ?

I'm not aware of this, will update the fix tags in the next patch.

> I wonder if it's simpler to just hold the netdev lock in resize or xsk
> binding instead of this.

That looks cleaner, let me try that approach.

Thanks,
Quang Minh